The invention relates to a method for producing deep cylindrical macropores in n-type silicon by means of electrochemical etching using illumination of the reverse side.
Like other semiconductors, silicon can be provided with pores by electrochemically dissolving. A basic classification according to IUPAC norm (International Union of Pure and Applied Chemistry) into three classes is carried out according to the size of the pores; a distinction is made here between microporous silicon (pore diameter<2 nm), mesopores (pore diameter 2-50 nm) and macropores (pore diameter>50 nm). However, the IUPAC definition only applies to the pore diameters; the distance between pores is not addressed thereby as a matter of principle despite in practice mostly having similar sizes as the pore diameter.
No binding nomenclature exists regarding the shape or morphology of the pores. Among experts it is generally known that micropores exhibit a spongy morphology while meso and macropores are directed. In the case of macropores in particular perfectly cylindrical (“channel-like”) pores can be produced, while mesopores rather have an angular (“coral-like”) morphology.
Potential applications for macroporous silicon are for example photonic crystals, micro filter diaphragms, optical filters, fuel cells, or biochips. One of the established methods for etching macropores having a defined geometry and arrangement in conventional n-type silicon wafers comprises the electrochemical dissolution using an aqueous (aqu) electrolyte containing hydrofluoric acid (HF) using backside illumination (bsi). To obtain a better reproducibility of the pore-etching results, the illumination is controlled here as a matter of principle such that a predetermined etching current flows. Further details can be gathered from the publications H. Föll, M. Christophersen, J. Carstensen, and G. Hasse, “Formation and application of porous silicon”, Mat. Sci. Eng. R 39(4), 93 (2002) and H. Föll, “Properties of silicon-electrolyte junctions and their application to silicon characterization”, Appl. Phys. A 53, 8 (1991).
U.S. Pat. No. 7,031,566 for example discloses a method for electrochemically etching macropores in n-type silicon, where the wafers are etched using an aqueous electrolyte solution containing hydrochloric acid while their reverse side is illuminated.
Certain demands are placed on etching these pores referred to below as n-macropores (aqu, bsi) according to the intended use that can be very detailed in the particular case but usually always exhibit three parameters:                1. obtainable maximum depth αmax of the etched pores,        2. etch time tetch up to the desired depth α, or, equivalent thereto, the mean etch rate νetch=α/tetch,        3. the roughness rp of the pore walls, measured e.g. using an atomic force microscope (AFM) as “root mean square” (rms) of the course of the surface profile.        
Further demands may exits beyond these, e.g. a pore diameter that is as constant as possible relative to the pore depth or a default of the cross-sectional shape of the pore (e.g. round—square). However these shall not be considered below.
A non-linear dependence exists between the three parameters mentioned above that is not fully understood to date. At most a few rules of thumb can be formulated so far, for example that high etch rates νetch and large maximum etch depths αmax represent “opposing” demands—if you want to etch deep, as a rule you have to etch slowly. Even in the case of very slow etching you will not be able to exceed certain etch depths. A simple relation in the form of νetch×αmax=const therefore also does not do justice to the complexity of the system. Something similar applies for the roughness rp of the pore walls. The assumption that is obvious per se that high etch rates cause rougher pore walls turns out not to be the case. Correlations between pore depth and pore-wall roughness may be assumed but are not known.
Given that today a need exists in practice for optimizing one or more of the three parameters mentioned there are no clear procedures. Moreover electrochemical pore etching is still limited, i.e. in the prior art these parameters do not progress beyond certain limits.
While fast growth is generally known for meso and micropores in n-type silicon, today's limit value for the etch rate for deep, cylindrical macropores amounts to about 1 micrometer/minute. Though it has already been attempted to increase the growth rate of conventional macropores using suitable, obvious measures, in particular by increasing the HF concentration or the temperature. However this only leads to higher growth rates in the initial phase and ultimately results in the loss of pore stability and the termination of pore growth (see FIG. 1).
The problem thus lies in the increase in the current component by the walls of a pore that have already formed; it is called the leakage current. The pore wall-surface that increases continually during etching causes a gradual shift in the ratio of leakage current from the pore walls to the photogenerated etching current from the pore tips toward larger values. On the one hand control of the pore geometry thereby becomes increasingly difficult as the pore depth grows. On the other hand the tendency can be observed when using a high HF concentration that from a certain pore depth it is above all the pore walls that are etched in the vicinity of the pore tips. The pore tips then widen—what ultimately destroys them—all pore tips grow together and form a so-called cavity. A low HF concentration in contrast leaves the pore walls stable for a longer time and permits to advance the pores into greater depths.
All experiences made so far cumulate in the rule that deep macropores having a good cylindrical geometry in n-type silicon can only be produced for HF concentrations in the range 2-5% and current densities between 1-10 mA/cm2. In the best case, fast growing cylindrical macropores are today possible for small depths in the range of a few micrometers up to about 100 μm; deep cylindrical macropores (in the longitudinal range 300 μm-600 μm) cannot be formed using a high growth rate.
A remarkable exception from the rule mentioned is the inventors' patent specification DE 10 2004 011 394 B3 in which can also be found some of the data from FIG. 1 of this description. In the method described there the pores are however not etched using back illumination, but by means of an avalanche breakthrough at the pore tip which requires very high etching voltages to be applied and also presupposes a very high HF concentration. In this way deep, cylindrical macropores can be formed using etch rates up to a surprising value of 8 micrometers/minute. This however occurs in a special “pore growth mode” that was described for the first time in DE 10 2004 011 394 B3 and that in particular permits no control over the arrangement or the spacing of the pores. It is rather high pore densities that are required here since it is exactly the close proximity of the pores that guarantees the stability of the pore walls against the highly concentrated electrolyte. The usefulness of this accelerated etching is insofar limited above all to producing perforated diaphragms having a high pore density.
The pore depth αmax that can be achieved as a maximum for n-macropores (aqu, bsi) at an HF concentration of around 5% by weight amounts to 400-500 micrometers. However special measures are to be planned here, e.g. lowering the temperature from around 20° C. to around 10° C. during etching and continually increasing the applied voltage from 0.5V to 0.6V. At the same time, a widening of the pore diameter has to be counteracted by systematically decreasing the etching current. Pore depths in the range mentioned up to 500 micrometers can generally not be achieved unless care is taken to optimize etch rate and pore-wall roughness at the same time. In the end, mean etch rates up to 1 μm/min and wall roughnesses in the range of about 50 nm rms will have to be expected.
The person skilled in the art knows that the etch process is very sensitive to very small deviations from optimum time curves. Usually the expedient process window is very small and has to be determined again from scratch on a case by case basis. On top of this admixtures to the electrolyte can be helpful, e.g. surfactants in small amounts or larger amounts of ethanol. In particular acetic acid is added often in small concentrations so as to improve wetting of the hydrophobic Si surface by the aqueous electrolyte. Basically only few details are known regarding this issue since as a rule they are not published.
To date there was little investigation of the pore-wall roughness; what are known are the results published by E. Foca, J. Carstensen, M. Leisner, E. Ossei-Wusu, O. Riemenschneider, and H. Föll, “Smoothening the pores walls in macroporous n-Si”, ECS Transactions, 211th Meeting of The Electrochemical Society, Chicago 6(2), 367 (2007). It is shown there that a reduction in the wall roughness—important for optical applications—can be achieved without the maximum pore depth suffering, in that different alcohols (methanol, ethanol, propanol) are added to the electrolyte, but in general this has the tendency to further reduce the etch rate.
For cost reasons one will always attempt to keep the etch duration tetch in the range of usual single-process times, i.e. a few minutes. However etch depths of 400-500 μm are today associated with etch times of tetch≈500-800 min. Therefore macroporous silicon can be utilized cost-effectively only to a limited extent.